An In-depth Discussion of M + Auto-ISO Part II

The reason I am writing this piece is not to convince everyone that they should convert to an auto-ISO user, but simply to explain why auto-ISO is a very good feature. Seeing that many people tend to dismiss this feature without really understand how to use it, and discourage others from trying it at the same time, I feel it calls for a proper discussion.

Introducing Auto-ISO

Auto-ISO is a feature that lets the camera change ISO automatically. The first canon SLR to have real auto-ISO is 7D. Canons before 7D had a non-working auto-ISO that was always stuck on ISO 400. Since 7D, auto-ISO would select the ISO that would give you correctly exposed final image based on the calculations from the camera's metering system, in combination with chosen shutter speed and aperture.

Personally I find auto-ISO to be the most useful in M mode, I'll explain why and then explain its application in Av mode. But before we do we will first discussion some surrounding issues.

The Nature of ISO

In digital photography, increasing ISO is increasing the level of amplification to the signal that sensor has received, and base ISO is simply leaving that signal free of any additional amplification. Amplification is either done by hardware (or sometimes called analog) or by software. Canon APS-C SLRs are only capable of hardware amplification at full stops, e.g. ISO 200/400/800 etc. 1/3 and 1/2 stop increments are done by in camera software and you can achieve the same effect in post processing.

Software amplification is manipulation to the image file after hardware amplification is finished; it does not achieve the same image quality as hardware amplification. The reduced quality can be demonstrated by a simple comparison. Shoot two images with the same shutter speed and aperture under the same lighting, one shot at ISO 100 and underexposed by 3 stops, then increase exposure by 3 stop in raw processing software, while the other shot at ISO 800 and is correctly exposed. It would be immediately apparent to the viewer that the ISO 800 image has much less noise as well as better contrast and colour accuracy than the other.

This is a very important point to remember, because it means by manually setting ISO, what you are dictating is the amount of amplification the camera will apply to the image after it is captured, whereas controlling aperture and shutter speed is controlling how much light is to be captured.

Earlier I have said that these in-camera 1/3 stop ISO increments are done by software and that software is not ideal. While this is true in reality 1/3 stop push or pull makes hardly any real impact to image quality, so for all intended purposes it can be overlooked.

On a side note, all current canon APS-C cameras has ISO’s upper limit set at ISO 6400. ISO 12800 is selectable but is software pushed from ISO 6400 and is not available in Auto-ISO. I do not know if it is due to limitations of Canon’s hardware amplification technology or that at this level, image quality is already destroyed so badly that hardware amplification would produce no better result than software.

Camera Metering

Regardless what some people would like to claim, the human eye and brain adapts to different lighting conditions way too well so that we are unable to correctly judge the actual amount of light surrounds us. Camera engineers recognised this long ago and invented electronic metering system. The metering system judges the scene for us and calculates the required shutter speed and/or aperture and/or ISO. If we use these calculated values to shoot the picture we will get a balanced looking image, we call it "correctly exposed image".

As of 2011, the metering system from the world's largest and best camera manufacture is very advanced and very accurate. Majority of camera users - those who use Av, Tv, P, or greenbox mode, are quite happily relying on the metering system. The only people that don’t use it are those who use M mode in its classic fashion (i.e. manual ISO). They rely on their "brain and experience" when it comes to metering. As far as I can see, they are not producing "more correctly exposed" images, and if I was forced to make a guess, I would say they produce a lot more unusable images than the rest of us when experimenting different settings to get the correct exposure in a given scene.

Now, if by using Av mode, where the user dictates aperture and ISO value, we trust the metering system to determine the correct shutter speed that would deliver a correctly exposed image, then there really is no reason as to where the user is to dictate aperture and shutter speed, why we would not trust the same metering system, using the same calculation formula, to determine the correct ISO that would deliver a correctly exposed image. That mode is M + auto-ISO.